M4: admin settings page and JSON API

- AdminApiController (admin only, CSRF-protected): status, create key,
  download recovery kit, confirm (statement accepted + exact phrase),
  backends with their usable auth mechanisms, add/test/remove locations,
  start backups, schedule, snapshots, browse, queue restores
- RestoreJob runs web-initiated restores in the background; backup and
  restore runs are kept apart in the run table
- admin section 'NG Backup' with a Vue 3 / @nextcloud/vue 9 page: key and
  recovery-kit flow (download, tick statement, 'Yes, I confirm'), location
  form built from files_external's parameter definitions, schedule, run
  list with polling, snapshot browser and restore with the three modes
- auth mechanisms that depend on a user session or on files_external's
  own stored credentials are not offered

API tested with curl on NC34 (status, backends, schedule validation,
confirmation refusal, browse, queued restore and backup completed by
cron); non-admins get 403, anonymous 401.

lib/Settings/Admin.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Settings;
+
+use OCP\AppFramework\Http\TemplateResponse;
+use OCP\Settings\ISettings;
+use OCP\Util;
+
+class Admin implements ISettings {
+	public function getForm(): TemplateResponse {
+		Util::addScript('ng_backup', 'ng_backup-admin');
+		Util::addStyle('ng_backup', 'ng_backup-admin');
+		return new TemplateResponse('ng_backup', 'admin');
+	}
+
+	public function getSection(): string {
+		return 'ng_backup';
+	}
+
+	public function getPriority(): int {
+		return 10;
+	}
+}